The present invention relates in general to an apparatus which is constructed and arranged for producing an “on-demand” semi-solid material for use in a casting process. Included as part of the overall apparatus are various stations which have the requisite components and structural arrangements which are to be used as part of the process. The method of producing the on-demand semi-solid material, using the disclosed apparatus, is included as part of the present invention.
More specifically, the present invention incorporates electromagnetic stirring and various temperature control and cooling control techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. Also included are structural arrangements and techniques to discharge the semi-solid material directly into a casting machine shot sleeve. As used herein, the concept of “on-demand” means that the semi-solid material goes directly to the casting step from the vessel where the material is produced. The semi-solid material is typically referred to as a “slurry” and the slug which is produced as a “single shot” is also referred to as a billet. These terms have been combined in this disclosure to represent a volume of slurry which corresponds to the desired single shot billet.
Semi-solid forming of light metals for net-shape and near-net shape manufacturing can produce high strength, low porosity components with the economic cost advantages of die casting. However, the semi-solid molding (SSM) process is a capital intensive proposition tied to the use of metal purchased as pre-processed billets or slugs.
Parts made with the SSM process are known for high quality and strength. SSM parts compare favorably with those made by squeeze casting, a variation of die casting that uses large gate areas and a slow cavity fill. Porosity is prevented by slow, non-turbulent metal velocities (gate velocities between 30 and 100 in./sec.) and by applying extreme pressure to the part during solidification. Both squeeze casting and SSM processes produce uniformly dense parts that are heat-treatable.
SSM offers the process economics of die casting and the mechanical properties that approach those of forgings. In addition, SSM capitalizes on the non-dendritic microstructure of the metal to produce parts of high quality and strength. SSM can cast thinner walls than squeeze casting due to the globular alpha grain structure, and it has been used successfully with both aluminum and magnesium alloys. SSM parts are weldable and pressure tight without the need for impregnation under extreme pressure that characterizes the squeeze-cast process.
The SSM process has been shown to hold tighter dimensional capabilities than any other aluminum molding process. That has intensified demand for SSM components due to the potential for significant cost savings, reduction of machining, and quicker cycle times for higher production rates. Besides high strength and minimal porosity, SSM parts exhibit less part-to-die shrinkage than die cast parts and very little warpage. It produces castings that are closer to the desired net shape, which reduces and can even eliminate secondary machining operations. Surface finishes on the castings are often better than the iron and steel parts they replace.
The SSM process requires higher final mold pressure (15,000 to 30,000 psi) than conventional die casting (7,000 to 12,000 psi), but modern die casting equipment provides the flexibility needed to produce SSM parts efficiently and economically. Real-time, closed-loop hydraulic circuits incorporated into today's die casting machines can automatically maintain the correct fill velocities of the SSM material alloy. Closed-loop process control systems monitor metal temperature and time, voltage feedback from electrical stator and other data to provide a very robust and precisely controlled operation that can maximize productivity of high quality parts and ensure reproducibility.
As described, it is well known that semi-solid metal slurry can be used to produce products with high strength and low porosity at net shape or near net shape. However, the viscosity of semi-solid metal is very sensitive to the slurry's temperature or the corresponding solid fraction. In order to obtain good fluidity at high solid fraction, the primary solid phase of the semisolid metal should be nearly spherical.
In general, semi-solid processing can be divided into two categories; thixocasting and rheocasting. In thixocasting, the microstructure of the solidifying alloy is modified from dendritic to discrete degenerated dendrite before the alloy is cast into solid feedstock, which will then be re-melted to a semi-solid state and cast into a mold to make the desired part. In rheocasting, liquid metal is cooled to a semi-solid state while its microstructure is modified. The slurry is then formed or cast into a mold to produce the desired part or parts.
The major barrier in rheocasting is the difficulty to generate sufficient slurry within preferred temperature range in a short cycle time. Although the cost of thixocasting is higher due to the additional casting and remelting steps, the implementation of thixocasting in industrial production has far exceeded rheocasting because semi-solid feedstock can be cast in large quantities in separate operations which can be remote in time and space from the reheating and forming steps.
In a semi-solid casting process, generally, a slurry is formed during solidification consisting of dendritic solid particles whose form is preserved. Initially, dendritic particles nucleate and grow as equiaxed dendrites within the molten alloy in the early stages of slurry or semi-solid formation. With the appropriate cooling rate and stirring, the dendritic particle branches grow larger and the dendrite arms have time to coarsen so that the primary and secondary dendrite arm spacing increases. During this growth stage in the presence of stirring, the dendrite arms come into contact and become fragmented to form degenerate dendritic particles. At the holding temperature, the particles continue to coarsen and become more rounded and approach an ideal spherical shape. The extent of rounding is controlled by the holding time selected for the process. With stirring, the point of “coherency” (the dendrites become a tangled structure) is not reached. The semi-solid material comprised of fragmented, degenerate dendrite particles continues to deform at low shear forces. The present invention incorporates apparata and methods in a novel and unobvious manner which utilize the metallurgical behavior of the alloy to create a suitable slurry within a comparatively short cycle time.
When the desired fraction solid and particle size and shape have been attained, the semi-solid material is ready to be formed by injecting into a die-mold or some other forming process. Primary aluminum (alpha) particle size is controlled in the process by limiting the slurry creation process to temperatures above the point at which solid alpha begins to form and alpha coarsening begins.
It is known that the dendritic structure of the primary solid of a semi-solid alloy can be modified to become nearly spherical by introducing the following perturbation in the liquid alloy near liquidus temperature or semi-solid alloy:                1) Stirring: mechanical stirring or electromagnetic stirring;        2) Agitation: low frequency vibration, high-frequency wave, electric shock, or electromagnetic wave;        3) Equiaxed Nucleation: rapid under-cooling, grain refiner;        4) Oswald Ripening and Coarsening: holding alloy in semi-solid temperature for a long time.        
While the methods in (2)-(4) have been proven effective in modifying the microstructure of semi-solid alloy, they have the common limitation of not being efficient in the processing of a high volume of alloy with a short preparation time due to the following characteristics or requirements of semi-solid metals:                High dampening effect in vibration.        Small penetration depth for electromagnetic waves.        High latent heat against rapid under-cooling.        Additional cost and recycling problem to add grain refiners.        Natural ripening takes a long time, precluding a short cycle time.        
While most of the prior art developments have been focused on the microstructure and rheology of semi-solid alloy, temperature control has been found by the present inventors to be one of the most critical parameters for reliable and efficient semi-solid processing with a comparatively short cycle time. As the apparent viscosity of semi-solid metal increases exponentially with the solid fraction, a small temperature difference in the alloy with 40% or higher solid fraction results in significant changes in its fluidity. In fact, the greatest barrier in using methods (2)-(4), as listed above, to produce semi-solid metal is the lack of stirring. Without stirring, it is very difficult and likely impossible to make alloy slurry with the required uniform temperature and microstructure, especially when the there is a requirement for a high volume of the alloy. Without stirring, the only way to heat/cool semi-solid metal without creating a large temperature difference is to use a slow heating/cooling process. Such a process often requires that multiple billets of feedstock be processed simultaneously under a pre-programmed furnace and conveyor system, which is expensive, hard to maintain, and difficult to control.
While using high-speed mechanical stirring within an annular thin gap can generate high shear rate sufficient to break up the dendrites in a semi-solid metal mixture, the thin gap becomes a limit to the process's volumetric throughput. The combination of high temperature, high corrosion (e.g. of molten aluminum alloy) and high wearing of semi-solid slurry also makes it very difficult to design, to select the proper materials and to maintain the stirring mechanism.
Prior references disclose the process of forming a semi-solid slurry by reheating a solid billet, formed by thixocasting, or directly from the melt using mechanical or electromagnetic stirring. The known methods for producing semi-solid alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for forming a slurry with the desired structure are controlled, in part, by the interactive influences of the shear and solidification rates.
In the early 1980's, an electromagnetic stirring process was developed to cast semi-solid feedstock with discrete degenerate dendrites. The feedstock is cut to proper size and then remelt to semi-solid state before being injected into mold cavity. Although this magneto hydrodynamic (MHD) casting process is capable of generating high volume of semi-solid feedstock with adequate discrete degenerate dendrites, the material handling cost to cast a billet and to remelt it back to a semi-solid composition reduces the competitiveness of this semi-solid process compared to other casting processes, e.g. gravity casting, low-pressure die-casting or high-pressure die-casting. Most of all, the complexity of billet heating equipment, the slow billet heating process and the difficulties in billet temperature control have been the major technical barriers in semi-solid forming of this type.
The billet reheating process provides a slurry or semi-solid material for the production of semi-solid formed (SSF) products. While this process has been used extensively, there is a limited range of castable alloys. Further, a high fraction of solids (0.7 to 0.8) is required to provide for the mechanical strength required in processing with this form of feedstock. Cost has been another major limitation of this approach due to the required processes of billet casting, handling, and reheating as compared to the direct application of a molten metal feedstock in the competitive die and squeeze casting processes.
In the mechanical stirring process to form a slurry or semi-solid material, the attack on the rotor by reactive metals results in corrosion products that contaminate the solidifying metal. Furthermore, the annulus formed between the outer edge of the rotor blades and the inner vessel wall within the mixing vessel results in a low shear zone while shear band formation may occur in the transition zone between the high and low shear rate zones. There have been a number of electromagnetic stirring methods described and used in preparing slurry for thixocasting billets for the SSF process, but little mention has been made of an application for rheocasting.
The rheocasting, i.e., the production by stirring of a liquid metal to form semi-solid slurry that would immediately be shaped, has not been industrialized so far. It is clear that rheocasting should overcome most of limitations of thixocasting. However, in order to become an industrial production technology, i.e., producing stable, deliverable semi-solid slurry on-line (i.e., on-demand) rheocasting must overcome the following practical challenges: cooling rate control, microstructure control, uniformity of temperature and microstructure, the large volume and size of slurry, short cycle time control and the handling of different types of alloys, as well as the means and method of transferring the slurry to a vessel and directly from the vessel to the casting shot sleeve.
One of the ways to overcome above challenges, according to the present invention, is to apply electromagnetic stirring of the liquid metal when it is solidified into semi-solid ranges. Such stirring enhances the heat transfer between the liquid metal and its container to control the metal temperature and cooling rate, and generates the high shear rate inside of the liquid metal to modify the microstructure with discrete degenerate dendrites. It increases the uniformity of metal temperature and microstructure by means of the molten metal mixture. With a careful design of the stirring mechanism and method, the stirring drives and controls a large volume and size of semi-solid slurry, depending on the application requirements. The stirring helps to shorten the cycle time by controlling the cooling rate, and this is applicable to all type of alloys, i.e., casting alloys, wrought alloys, MMC, etc.
While propeller type, mechanical stirring has been used in the context of making a semi-solid slurry, there are certain problems or limitations. For example, the high temperature and the corrosive and high wearing characteristics of semi-solid slurry, makes it very difficult to design a reliable slurry apparatus with mechanical stirring. However, the most critical limitation of using mechanical stirring in rheocasting is that its small throughput cannot meet the requirements production capacity. It is also known that semi-solid metal with discrete degenerated dendrite can also be made by introducing low frequency mechanical vibration, high-frequency ultra-sonic waves, or electric-magnetic agitation with a solenoid coil. While these processes may work for smaller samples at slower cycle time, they are not effective in making larger billet because of the limitation in penetration depth. Another type of process is solenoidal induction agitation, because of its limited magnetic field penetration depth and unnecessary heat generation, it has many technological problems to implement for productivity. Vigorous electromagnetic stirring is the most widely used industrial process permits the production of a large volume of slurry. Importantly, this is applicable to any high-temperature alloys. The present invention, which focuses on the apparata and methods of delivering a semi-solid slurry on demand, employs the use of multiple-pole stators.
Two main variants of vigorous electromagnetic stirring exist, one is termed “rotary ” stator stirring due to the rotary flow pattern of the alloy within the vessel. The other is termed “linear” stator stirring due to the up and down flow loop of the alloy within the vessel. With rotational or rotary stator stirring, the molten metal is moving in a quasi-isothermal plane, therefore, the degeneration of dendrites is achieved by dominant mechanical shear. U.S. Pat. No. 4,434,837, issued Mar. 6, 1984 to Winter et al., describes an electromagnetic stirring apparatus for the continuous making of thixotropic metal slurries in which a stator having a single two pole arrangement generates a non-zero rotating magnetic field which moves transversely of a longitudinal axis. The moving magnetic field provides a magnetic stirring force directed tangentially to the metal container, which produces a shear rate of at least 50 sec−1 to break down the dendrites. With linear stator stirring, the slurries within the mesh zone are re-circulated to the higher temperature zone and remelted, therefore, the thermal processes play a more important role in breaking down the dendrites. U.S. Pat. No. 5,219,018, issued Jun. 15, 1993 to Meyer, describes a method of producing thixotropic metallic products by continuous casting with polyphase current electromagnetic agitation. This method achieves the conversion of the dendrites into nodules by causing a refusion of the surface of these dendrites by a continuous transfer of the cold zone where they form towards a hotter zone.
A part formed according to this invention will typically have equivalent or superior mechanical properties, particularly elongation, as compared to castings formed by a fully liquid-to-solid transformation within the mold, the latter castings having a dendritic structure characteristic of other casting processes.